1. Field of the Invention
The present invention relates to a fault detection system for detecting faults occurring in an inverter, e.g., three-phase PWM inverter.
2. Description of the Related Art
An exemplary conventional fault detection system used for a three-phase power converter (three-phase inverter) is found in Japanese Patent No. 2902455 (Publication No: JP-A-4-87553). The fault detection system is provided with a detector, a sample hold circuit, a multiplexer, an A/D converter, and a micro processor. Specifically, the detector is provided to detect three-phase voltage and current relating to the three-phase power converter, and analog signals of the three-phase voltage and current detected by the detector are sample-hold by the sample hold circuit. The analog signals of the three-phase voltage and current are also output from the multiplexer responsively to the sequential selection made on a signal basis. Thus selected analog signals are sequentially forwarded to the A/D converter to be converted therein into digital values for output. The microprocessor reads in the digital values as a result of A/D conversion in the A/D converter, and uses the values for control operation. The voltage values and the current values read in by the microprocessor are each summed on a phase basis. When the resulting values fall in a certain range with a center value of 0, the fault detection system detects it as normal. If the resulting values do not fit in the range, it is detected as abnormal.
As such, in the conventional fault detection system for a three-phase inverter as found in the above patent document, fault determination is made by subjecting instantaneous values of the three phase voltage and current to A/D conversion through the sample hold circuit, and by detecting whether the sum of the A/D-converted values is in the vicinity of 0. This is applicable to the instantaneous values of three phase currents.
The problem here is that, due to the above-described structure, such a conventional fault detection system for a three-phase inverter requires a high-speed A/D converter to monitor instantaneous values of an inverter output voltage having a rectangular waveform.
What is more, with a three-phase inverter low in bus voltage for in-car use, to have a higher usage rate for the bus voltage, the three-phase inverter may be controlled to derive sinusoidal line voltage, i.e., inter-phase voltage.
If this is the case, the sum value of the phase voltages does not become 0, and thus no fault detection is available for the three-phase inverter.
Herein, the bus voltage is a direct current voltage to be applied to the three-phase inverter for conversion into an alternating voltage. Here, with a three-phase inverter for in-car use, the bus voltage is a battery voltage.